The present invention relates to a high purity tantalum sputtering target comprising a uniform and fine structure and which yields stable plasma and yields superior film evenness, in other words, uniformity. Note that, the generic term of “high purity tantalum” will be used in the present specification since the high purity tantalum according to the present invention contains (is added with) tungsten, and molybdenum and/or niobium as needed, and the additive amount of these elements is small.
In recent years, the sputtering method for forming a film from materials such as metal or ceramics has been used in numerous fields such as electronics, corrosion resistant materials and ornaments, catalysts, as well as in the manufacture of cutting/polishing materials and abrasion resistant materials.
Although the sputtering method itself is a well-known in the foregoing fields, recently, particularly in the electronics field, a tantalum sputtering target suitable for forming films of complex shapes, forming circuits or forming barrier films is in demand.
Generally, this tantalum target is produced by repeating the hotforging and annealing (heat treatment) of an ingot or billet formed by performing electron beam melting and casting to a tantalum raw material, and thereafter performing rolling and finish processing such as mechanical processing and polishing thereto.
In this kind of production process, the hot forging performed to the ingot or billet will destroy the cast structure, disperse or eliminate the pores and segregations, and, by further annealing this, recrystallization will occur, and the densification and strength of the structure can be improved to a certain degree.
The molten and cast ingot or billet generally has a crystal grain size of 50 mm or more. As a result of subjecting the ingot or billet to hot forging and recrystallization annealing, the cast structure is destroyed and uniform and fine (100 μm or less) crystal grains as a whole can be obtained.
Meanwhile, if sputtering is to be performed using a target produced as described above, it is said that the recrystallized structure of the target becomes even finer and more uniform, more uniform deposition is possible with a target in which the crystal orientation is aligned toward a specific direction, and a film with low generation of arcing and particles and stable characteristics can be obtained.
Thus, measures are being taken for achieving a finer and more uniform recrystallized structure and aligning the crystal orientation toward a specific direction in the production process of the target (for example, refer to Patent Document 1 and Patent Document 2).
Moreover, disclosed is a high purity Ta target for forming a TaN film to be used as a barrier layer in a Cu wiring film which is obtained by containing 0.001 to 20 ppm of an element selected among Ag, Au and Cu as an element having self-sustained discharge characteristics, causing the total amount of Fe, Ni, Cr, Si, Al, Na, and K as impurity elements to be 100 ppm or less, and using a high purity Ta in which the value obtained by subtracting such impurity elements is within the range of 99.99 to 99.999% (refer to Patent Document 3).
When reviewing these Patent Documents, there is no disclosure to the effect of the inclusion of a specific element realizing a finer structure and thereby stabilizing the plasma.
Particularly, Patent Document 3 increases the discharge amount of Ta ions by adding an infinitesimal amount of an element up to 0.001 ppm as a result of containing an element selected among Ag, Au and Cu in an amount of 0.001 to 20 ppm. However, since the additive element is a trace amount, it is considered that there is a problem of difficulty in adjusting the content and in adding evenly (variation).
As shown in Table 1 of Patent Document 3, the inclusion of amounts of Mo, W, Ge, and Co is respectively tolerable at less than 10 ppm, 20 ppm, 10 ppm, and 10 ppm. This alone adds up to impurities in an amount that is less than 50 ppm.
Accordingly, as described above, although Patent Document 3 describes “causing the total amount of Fe, Ni, Cr, Si, Al, Na, and K as impurity elements to be 100 ppm or less, and using a high purity Ta in which the value obtained by subtracting such impurity elements is within the range of 99.99 to 99.999%,” the lower limit of the actual purity falls below (tolerates) 99.99%.
This is a level that is lower than conventional high purity tantalum, and it is strongly assumed that the characteristics of high purity tantalum cannot be utilized.    [Patent Document 1] Published Japanese Translation of WO2002-518593    [Patent Document 2] U.S. Pat. No. 6,331,233    [Patent Document 3] Japanese Published Unexamined Patent Application No. 2002-60934